| In recent years,with the international and domestic requirements for the continuous improvement of emissions,the engine's thermal efficiency has a higher demand,the engine cylinder exhaust temperature is further increased.Nomonic 80A and Inconel 751 two nickel-based alloy maximum operating temperature of 730 ? or so,the temperature is further increased,the two nickel-based alloy will appear creep elongation and high temperature strength of the situation.In order to meet the needs of the engine for higher exhaust temperatures,the article aims to launch a new type of valve alloy LF8 that can operate at about 750 ? to replace the currently used Nimonic 80A.Firstly,Thermo-Calc thermodynamics simulation software was used to optimize the design components.The effects of different alloying elements on the equilibrium precipitation phase were studied and the range of design components was determined.According to the design composition smelting test steel,after the use of electron microscopy,transmission electron microscopy,micro-phase analysis and mechanical properties of the test method and means of LF8 alloy heat treatment process,long-term aging after the study of the changes in microstructure and mechanical properties,the result shows that:LF8 alloy matrix is austenite,the main precipitation phase is ?' phase,M23C6,M7C3 phase,a-Cr phase and M6C phase;With the increase of Cr,Al and Ti content,the carbide type was changed from M7C3 to M23C6,and then the number of M23C6 increased with the increase of the three elements,When the Cr content exceeds 20%,the a-Cr phase precipitates in large amounts;LF8 alloy design component range,Cr:17.0?19.0%,A1:2.0?3.0%,Ti:3.5?4.0%,Co:2.0?4.0%,Fe:5.0?7.0%.The results show that:The best heat treatment system of LF8 alloy is 1050 ?solid solution + 750 ? aging;LF8 alloy treated by the best heat treatment system to y as the matrix,precipitation phase ?',Cr23C6 and TiC phase,Where the ?' phase is the most important strengthening phase,the small dispersion of the distribution in the y matrix,the size of about 20 nm;Cr23C6 phase in the grain boundary precipitation,oval,the size of about 400?800 nm;TiC phase in the crystal and grain boundary near a small amount of precipitation,was massive,about 400 nm in size,mainly to the primary TiC;LF8 room temperature tensile strength,yield strength and hardness than 80A were higher than 8.6%,20%and 7.9%;750 ? tensile strength and yield strength were higher than 80A 5%and 12.4%;The precipitates of the alloy during the long aging process at 750 ? have ?' phase,Cr23C6 and TiC phases,With the extension of the aging time,the spherical ?' phase content increased,the size increases;With the prolongation of long-term aging time,Cr23C6 content increased,the size increased from the independent fine precipitation to semi-continuous chain distribution in the grain boundary;1000h is still semi-continuous state,indicating that LF8 has a good long-term stability of the organization;LF8 alloy at 750?,100h creep after the creep plastic elongation of only 80A 55%,high temperature performance is better than 80A and there is hope to become a 750 ? valve alloy alternative materials. |
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